Television and monitor displays may be subject to high luminance signals. As a result, when there is higher saturation of color, higher luminance, or larger amplitude in a video signal than desired, the linearity of reproduction of color signals may be sacrificed. This leads to an undesirable color saturation (color distortion) on the display screen. To solve this problem, a primary color video signal output circuit is disclosed, for example, in Japanese Laid-open Patent No. 55-46688.
FIG. 10 is a block diagram showing a portion of one channel of a conventional video signal output circuit composed of plural primary color signal output circuits.
In FIG. 10, a primary color video signal 101 is applied to a clip circuit composed of a Zener diode 103. An inverting amplifier composed of a power supply unit 104, a resistance 105, a transistor 106, and a resistance 107, through a circuit 102 aligns the black level of each primary color video signal for the display element (not shown), and adjusts white balance of the signal at specified brightness (hereinafter called cutoff/drive adjusting circuit).
The clip circuit is for clipping the signal at a specific constant level, before inputting the signal into the inverting amplifier, so as to not spoil the linearity of the output signal, by not excessively driving the inverting amplifier. Excessive driving of a succeeding video signal processing circuit and a succeeding display element are also prevented.
The operation of the video signal output circuit is described below.
FIG. 11A, FIG. 11B, and FIG. 11C show a conceptual diagram of an input signal and an output signal of the video signal output circuit. More specifically, FIG. 11A shows a signal waveform diagram when the clip circuit is not used, and FIG. 11B shows an expected signal waveform diagram when the clip circuit is used.
The clip circuit clips the video signal so that the succeeding video signal processing circuit and display element may not be driven excessively.
The saturation level LS shown in FIG. 11A and FIG. 11B refers to the peak value of maximum signal voltage that can be processed by the inverting amplifying circuit, succeeding video signal processing circuit, and display element. Clipping is done at a level within this saturation level LS.
The cutoff level LC refers to the black level of display element.
Initially, the Zener diode was used in the clip circuit for the purpose of utilizing the feature produced by the Zener characteristic, that is, when exceeding the Zener voltage, the Zener current is raised very violently, so that a constant voltage is obtained.
In this circuit, however, depending on the characteristic of the Zener diode, the picture is blunted near the clip level.
Individual Zener diodes are not always identical in characteristics. Therefore, clipping may differ depending on the individual difference of characteristics of Zener diodes.
In commercial Zener diodes, the inverse bias voltage, when passing a specified current to the diode, is controlled very well. A specified narrow width of a voltage is shown. However, in the region of current smaller than the specified current, the diode inverse bias voltage is not controlled, and the fluctuation width of the voltage is wide. Although the leak current of the Zener diode is a lower value than the specified current, some current is flowing and is not controlled. This is because the leak current of the Zener diode may not be an important control item in manufacture as long as this value is below the specified value.
When a Zener diode is used in a clip circuit, the leak current increases before the Zener potential of the Zener diode is reached. The waveform of the primary color video signal 101 begins to be blunted at a lower voltage than actually clipped. Thus, linearity of the signal is broken. FIG. 11C shows a blunt profile of a wave form when clipped by the Zener diode. Ideally, the primary color video signal 101 is clipped in the portion higher than the Zener voltage.
Depending on the variations of characteristics of the Zener diodes being used, however, the manner of collapse of linearity of the video signal differs in each channel, which may possibly lead to collapse of linearity of the white balance signal in the high luminance region.